The effect of spatial contrast on response gain of the human visual system will be studied using corneal (ERG) and scalp (VEP) recordings and by evaluating related psychophysical functions. First, "transient" and "steady state" ERG components which can be attributed to spatial contrast and not simply to rectified local luminance responses will be identified. Sequential processing stages and areal interactions of the human retina will be evaluated from amplitude and phase properties of linear and nonlinear (interactive) steady-state response components which occur when two temporal frequencies of modulation are simultaneously applied to a grating pattern. The effect of surround and overlap contrast masking on both luminance and pattern responses will be studied electrophysiologically and psychophysically. Our studies will answer whether or not there is a retinal mechanism of surround masking. If present, differences in surround and overlap masking may be advantageously used to contrast retinal and cortical networks, as it has been suggested some time ago that different types of masking may occur at different levels of the visual system. We will answer the question of whether or not a spatial, as separate from temporal, nonlinearity precedes the surround "masking" stage. At the moment, the effect of spatial nonlinearities on masking is unknown either from psychophysical or VEP studies. Understanding the gain control of contrast on retinal responses is relevant to patients who suffer from Parkinson's disease (PD). Previous studies suggest impaired modulatory effect of dopamine (DA) on contrast responses in different species, including man. The proposed studies will evaluate if DA in the human affects contrast gain via an intraretinal feedback loop. These studies will reveal new pathophysiological concepts of PD. Analyzing interactive frequency responses, we wish to discern the effect of partial or complete scotoma on areal interactions. These may be defective in early glaucoma, perhaps prior to manifest visual field changes. Our previous studies suggest impaired low spatial frequency responses in glaucoma, while animal studies demonstrate abnormal magnocellular functions in this disease. The present studies will describe nonlinear visual responses in glaucoma and ocular hypertension (OHT). These studies may help to identify visual changes in OHT without visual field defects and thus contribute to a better understanding of the pathophysiology and early diagnosis and treatment of glaucoma.